The present invention relates to a process for removing skins from avocados and more particularly to such a process involving thermal treatment of the avocados to result in skin removal.
Avocados are similar to other foods including fruits and vegetables such as tomatoes and peaches, at least to the extent that the skins must generally be removed before conversion to a processed food product. However, the processing of avocados to remove their skins has not developed at the same pace as other such foods because of the thermal sensitivity of the avocado flesh.
By contrast, in the processing of food products such as tomatoes and peaches, they are commonly immersed in hot or boiling water in order to facilitate removal of the skins by conventional techniques which are well known in the prior art. Such a technique is particularly adapted for use with these food products since the flesh portion of tomatoes and peaches is intentionally cooked during processing. In any event, heating of such foods has no deleterious effects and may be considered an improvement in terms of flavor, texture, color or otherwise. On the other hand, as will be discussed in greater detail immediately below, such processing techniques have not proven satisfactory with avocados because of thermal sensitivity of the avocado flesh. Such thermal sensitivity is particularly noticeable in the green or chlorophyll layer of the avocado flesh because it lies just beneath the avocado skin and is thus subject to greatest exposure to heat transferred through the skin. Since much of the flavor and nutritional value of the avocados is found in this layer, it is particularly desirable that the green chlorophyll layer of the flesh be maintained in fresh condition to enhance both the flavor and color of resulting food products. However, since the green chlorophyll layer also lies immediately adjacent the skin, it is particularly subject to thermal degradation if the skin is removed by conventional thermal techniques.
Many different varieties of avocados are commercially important, including the Fuerte, Hass, Bacon, Reed, Zutano and Pinkerton varieties. The Fuerte and Hass varieties are probably of greatest commercial importance in California where avocado orchards are most prevalent.
Although there are some differences in these varieties, all avocados tend to be characterized by skins which are relatively tough or leather-like and which resist separation from the avocado flesh. Other characteristics of avocado skins many vary, for example, from the relatively thin smooth skin of the Zutano variety to the relatively thick and pebbly skin of the Hass variety.
The fruit or flesh of the avocado is relatively unique since it accumulates a large lipid reserve during development or ripening, thus providing an important potential source of dietary fat without the presence of undesirable cholesterol. Also, the avocado is rich in vitamin B, particularly pyridoxine and pantothenic acid while providing a reasonable amount of vitamin E (alpha-tocopherol). The mineral content of the avocado is also higher than other fresh fruit, particularly for phosphate, iron and potassium which are of substantial nutritional importance.
Thus, because of its high nutritive value and unusual composition, the avocado fruit has special value as a food product. However, in view of its characteristics as outlined above, the avocado is most useful in a fresh or fresh-frozen form. Because of the relative expense and difficulty in removing avocado skins in the past, the level of use of avocados in various food products has heretofore been limited. It is believed that market demand would be substantially greater is the avocado flesh were readily available with the skin and pit removed and absent thermal off-flavors and the like.
In the past, as was noted above, it has been well known that the heating of avocado flesh tends to develop off-flavors, thus precluding thermal processing to facilitate skin removal. The occurrence of off-flavors in thermally processed avocados has been well-documented, for example, in a 1951 review by Cruess, et al (Cruess, W. V., Gibson, A. and Brekke, J. F., Canner 112[2]:11:112[3]). R. P. Bates also discussed heat-induced off-flavor development during the processing of ripened avocado flesh (Journal of Food Science 35, 478, 1970). Referring to the above article by Bates, it is noted that, "The avocado, in contrast to most food crops, cannot be subjected to thermal processing or to air-drying without the production of off flavors. The deleterious flavor changes are characterized by an intense `blanched` aroma, a disagreeable pungent mouth-coating taste and an unpleasant lingering after-taste which, in many avocado varieties, is accompanied by bitterness."
Also, according to Ben-Et Dolev. A. and Tatarsky, O. (Journal of Food Science 38, 546-547, 1973), the blanching of avocados causes heat-induced production of 1-acetoxy-2,4-dihydroxyl-n-heptadec-16-ene and similar compounds which are characterized by a bitter taste.
For these reasons, the processing of avocados has heretofore been limited by relative expense and difficulty in removing the avocado skins. In many operations, the skins have been removed manually, not only resulting in an expensive and time-consuming operation, but also being characterized by the loss of part of the desirable green chlorophyll layer just beneath the avocado skin.
Mechanized techniques for removing avocado skins have been developed in the prior art but only with accompanying limitations. For example, one such technique involved halving of the avocado fruit and removal of the pit, the resulting avocado halves being pressed between two surfaces to extrude the flesh through perforations in one of the surfaces. Although this process overcame the expense of manual operations, it necessarily resulted in the recovered avocado flesh being extruded or mashed. By contrast, it is desirable in certain food products that the avocado flesh be present in a more textured form. At the same time, it is again noted that the desirable green chlorophyll layer of the avocado flesh lies immediately adjacent to the avocado skin and it is not known if complete recovery of this layer was possible by the above process.
In any event, there has been found to remain a need for an improved method of processing avocados to permit effective and efficient removal of the avocado skins, preferably while allowing recovery of all or substantially all of the green chlorophyll layer of the avocado flesh just beneath its skin. At the same time, it is necessary to avoid the introduction of thermally developed off-flavors in the avocado flesh.